Our research focuses on iron proteins that are involved in electron transfer, reversible oxygen binding and/or enzymatic reactions. Specifically, we plan to study horseradish peroxidase, lactoperoxidase, choloroperoxidase from the mold Caldariomyces fumago, catalase from Micrococcus luteus, cytochrome P450CAM and cresol hydroxylase from Pseudomonas putida, the terminal cytochrome oxidase from yeast and a cytochrome d oxidase from E. coli, hemerythrin, and purple acid phosphatase from pig allantoic fluid and from beef spleen. All these systems have an active center that contains iron and is reponsible for the specific function of the protein. The goal is to elucidate the properties of these active centers by various physical techniques and to correlate them with the biological function. Experiments are done in close collaboration with biochemists actively working on these system. We use 57Fe Mossbauer spectroscopy, EPR and ENDOR to probe the electronic state of the iron and its interaction with ligands. From a combination of measurements taken under different conditions we can characterize the electronic state of the acive center at different stages of the enzymatic cycle and attempt to model the reaction mechanism. Whenever possible the proteins are enriched in 57Fe for the Mossbauer studies, and other isotopic enrichments are used for ENDOR experiments. We plan to extend earlier ENDOR studies on cytochrome P450CAM to single crystals to gain insight into the geometry of the heme complex. We are particularly interested also in the analysis of exchange coupling between half integer and integer spin systems as observed in the porphyrin radical/ferryl complex of peroxidase primary compounds, or in the binuclear iron complexes of semimethemerythrin and purple acid phosphatase.